Lens design to eliminate color fringing

ABSTRACT

Projection lenses and projection lens systems are telecentric between an illumination subsystem and a set of imagers. The lenses and systems can exhibit color fringing correction, uniform imager illumination, athermalization, and component articulation for improved imaging. The lenses and systems may be employed in displays, such as folded displays that have decreased footprint size, but long effective projection lengths.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to projection lenses and projectionsystems, and, more particularly, to projection lenses and systems thatprovide improved use of the total light energy emitted by anillumination subsystem.

2. Description of Related Art

Light projection is used to display images on large surfaces, such aslarge computer displays or television screens. In front projectionsystems, an image beam is projected from an image source onto the frontside of a reflection-type, angle transforming screen, which reflects thelight toward a viewer positioned in front of the screen. In rearprojection systems, the image beam is projected onto the rear side of atransmission-type, angle transforming screen and transmitted toward aviewer located in front of the screen.

In single exit pupil projection systems, three primary color images areprojected through the same lens to form a full color image. Thesesystems avoid color shift in the projected image and color mixing orcombining need not be performed by their screen as in a three lenssystem. Single exit pupil systems may be either of the transmissivevariety or of the reflective variety. Additional information aboutprojection lenses and systems can be found in U.S. Pat. No. 5,218,480,issued to Moskovitch, entitled “Retrofocus Wide Angle Lenses,”incorporated by reference herein in its entirety.

Several considerations stand out for such projection systems. One itemis the efficient use of the light energy output of an illuminationsubsystem in a projection system. Matching the illumination subsystemwith imagers (e.g., a liquid crystal display (LCD) or spatial lightmodulator (SLM)) in the projection system to obtain a bright, uniformlyilluminated image is important. Etendue considerations have not beenparticularly emphasized in previous projection system designs. Examplesof the type of light sources in illumination subsystems, amongst others,for which efficiency can matter include metal-halide lamps and thosedescribed in U.S. Pat. Nos. 5,404,076 and 5,606,220, issued to Dolan etal., entitled “Lamp Including Sulfur” and “Visible Lamp IncludingSelenium or Sulfur,” respectively, and in U.S. Pat. No. Re. 34,492,issued to Roberts, entitled “Combination Lamp and Integrating Sphere ForEfficiently Coupling Radiant Energy From A Gas Discharge Into ALightguide.” U.S. Pat. Nos. 5,404,076, 5,606,220, and Re. 34,492 areincorporated by reference herein in their entirety. Other examplesinclude lamps described in PCT Pat. application No. PCT/US97/10490, byMacLennan et al., published as WO 97/45858 on Dec. 4, 1997, alsoincorporated by reference herein in its entirety.

Another consideration is system size. For rear projection and computerscreen applications, a small overall package size is desirable exceptperhaps for the screen. The physical size of individual components, suchas lenses, filters, stops, etc., should be made relatively small while alarge image size should be produced. Although a system may be small insize, however, its compactness may not necessarily be optimized. Forinstance, in projection systems employing three LCD imagers, one foreach primary color, the distance between the projection lens and theimagers may have to be increased to accommodate field lenses required tobetter match the illumination subsystem and the imagers.

In some previous projection lenses, the filtering of image or imagerillumination light has been of concern. A filter could be placed, forexample, within an aperture stop of a projection lens. However, aperturestops have previously been disadvantageously positioned within thephysical confines of one of the lenses or other elements making up theprojection lens.

Thermal effects have been a concern when polymer materials, despitetheir generally good optical properties, are used to constructindividual lens elements in projection lens systems. Aspheres, althoughuseful in limiting lens aberrations and in reducing lens size, canreveal detrimental thermal effects with high power light when positivelypowered optical elements are constructed of these materials. Acrylicmaterials, for example, present a relatively large change in refractiveindex with temperature. A lens fashioned out of acrylic can, therefore,display an internal temperature change or gradient. A correspondingoptical power change can result with high powered light, leading toperformance deficiencies.

Other considerations in projection systems include the effects ofdispersion in optical elements and manufacturing tolerances. Dispersioneffects frequently appear in optical systems in which all three primarycolors are transmitted through the same optical elements. Manufacturingtolerances can impact parts interchangeability. Manufacturing tolerancesmay result in performance variations that need to be addressed byappropriate means to ensure that production model projection lenses andsystems will demonstrate similar performance.

The present invention is directed to improving projection lenses andsystems. The present invention is also directed to overcoming orreducing one or more of the problems and deficiencies set forth above orother problems and deficiencies.

SUMMARY OF THE INVENTION

In general, in one aspect, embodiments of the invention feature aprojection lens apparatus that includes a front lens unit, a back lensunit, and a linear polarizer adapted to direct illumination light to theback lens unit and to direct image light to the front lens unit. Theprojection apparatus also includes a chromatic separating elementadapted to separate the illumination light into color separated light, afield lens adapted to change the magnification of a portion of the colorseparated light, and an imager adapted to direct the portion of thecolor separated light as image light.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to thedrawings in which:

FIG. 1 is a perspective view of a projection lens system in accordancewith a first embodiment of the invention.

FIG. 2 is a top view of the projection lens system in FIG. 1.

FIG. 3 is a more detailed top view of the projection lens system in FIG.1

FIG. 4 is a detailed view of a portion of a projection lens system inaccordance with an exemplary embodiment of the invention.

FIG. 4A provides a key between element surfaces and reference numeralsin FIG. 1.

FIG. 5 is a view of a projection lens system in accordance with a secondembodiment of the invention.

FIGS. 6A and 6B are detailed views of a portion of a projection lenssystem in accordance with an exemplary embodiment of the projection lenssystem in FIG. 5.

FIG. 7 is a view of a projection lens system with an illuminationsubsystem including an illumination relay lens system in accordance withan exemplary embodiment of the invention.

FIG. 8 is a view of a projection lens system with an illuminationsubsystem including an illumination relay lens system in accordance witha third embodiment of the invention.

FIG. 8A provides a key between element surfaces and reference numeralsin FIG. 8.

FIGS. 9 and 10 are views of portions of a projection lens system with anillumination subsystem in accordance with exemplary embodiments of theinvention.

FIGS. 11-14 are views of mounting apparatuses in accordance withexemplary embodiments of the invention.

FIGS. 15A and 15B are views of details of a projection lens system withan illumination subsystem in accordance with an alternative embodimentof the invention.

FIGS. 16 and 17 are views of details of portions of a projection lenssystem in accordance with exemplary embodiments of the invention.

FIGS. 18 and 18A are views of a portion of a projection lens system inaccordance with a fourth embodiment of the invention.

FIG. 19 is a side view of a display apparatus in accordance with a fifthembodiment of the invention.

FIG. 20 is a side view of another display apparatus in accordance with asixth embodiment of the invention.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers′ specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a developmenteffort, even if complex and time-consuming, would be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

Improved projection lenses and an improved projection lens systems aredescribed in accordance with embodiments of the invention. Theprojection lenses and systems have utility in both front and rearprojection systems. The projection lens systems can include illuminationand relay lens subsystems. The projection lenses and systems may beemployed advantageously in so-called “folded” optical display systems.In the description and drawings of the projection lenses and systemsbelow, like reference numerals are indicative of like parts.

FIGS. 1-3 illustrate a reflection-based projection lens system 10 inaccordance with a first embodiment of the invention. The projection lenssystem 10 includes a projection lens 12 having a first or front lensunit 14 and a second or back lens unit 16. The front lens unit 14 andthe back lens unit 16 are separated by an air gap. The front lens unit14 has overall zero, near-zero or weak (e.g., negative) optical powerwith an angular magnification to project over a wide field of view.Other embodiments can have positive or negative optical powers for thefront lens unit 14. The second lens unit 16 has overall positive opticalpower. In the exemplary embodiment shown in FIGS. 1-3, the second lensunit 16 includes lens elements 18, 20, and 22 and the first lens unit 14includes lens elements 24, 26, 28, and 30. The lens elements 18, 20, 22,24, and 26 are all positively powered lenses and the lens elements 28and 30 are both negatively powered lenses. The lens elements 18, 20, and24 may be doublets and the lens elements 22, 26, 28 and 30 may bemeniscus lenses, although other lens types or powers could be used.Other arrangements and number of elements can be envisioned, as will beappreciated by those skilled in the art having the benefit of thepresent disclosure. These other arrangements and number of elements areincluded within the scope and spirit of the present invention.

FIG. 3 shows a larger view of the projection lens system 10 and theprojection lens 12. The projection lens 12 includes nine elements in theexemplary embodiment. These nine elements include a reflecting linearpolarizer 32 in addition to the lens elements 18, 20, 23, 24, 26, 28,and 30, and a clean-up element 34 (see FIGS. 2 and 3). In otherembodiments, the number of elements can be other than nine. The clean-upelement 34 can be an absorptive linear polarizer and is optional. Thereflecting linear polarizer 32 may be constructed of double brightnessenhancement film (DBEF), a variety of multilayer optical film (MOF),commercially available from Minnesota, Mining & Manufacturing Company.The reflecting linear polarizer 32 (e.g., MOF) is a substantiallynonabsorbing polarizer. Hence, it does not substantially absorb lightthat it transmits or reflects. An exemplary construction of thereflecting linear polarizer 32 is a sandwich of glass, optical cement,MOF, optical cement, and glass. The reflecting linear polarizer 32 isoriented to substantially reflect first linear polarization componentsof light of desired colors (which can be white light or substantiallywhite light) from a light source (not shown in FIGS. 1-3) toward therear lens unit 16 and to substantially transmit second linearpolarization components (orthogonal to the first) and reflect undesiredcolors. For example, the reflecting linear polarizer 32 can be orientedwith its high efficiency side toward the light source, such thatincoming S polarization light is reflected and P polarization light istransmitted. An additional reflecting linear polarizer (not shown)constructed of MOF, for example, may be placed in the path of thetransmitted light to reflect it back through the reflecting linearpolarizer 32 to the light source. Such operation is useful with certaintypes of high intensity discharge (HID) lamps (to be described in moredetail below) or other types of lamps for optical “pumping” of the lightsource to improve the efficiency of the light source for generating thedesired light components. These lamps are exemplified in prior co-ownedU.S. patent application Ser. No. 08/747,190, filed Nov. 12, 1996, byRichard M. Knox, entitled “High Efficiency Lamp Apparatus For ProducingA Beam Of Polarized Light,” and Ser. No. 08/771,326, filed Dec. 20,1996, by William B. Mercer, entitled “Polarized Light Producing LampApparatus That Uses Low Temperature Polarizing Film,” both incorporatedby reference herein in their entirety. A remote aperture stop 33 islocated near the lens element 24 between the reflecting linear polarizer32 and the lens element 24, as shown in FIGS. 2 and 3. By positioningthe aperture stop 33 remotely from the polarizer 32 (i.e., by it beingan accessible aperture stop), diffractive and/or other out-of-anglelight can effectively be blocked from images. As a result, contrast canbe improved by pupil apodization for contrast enhancement and/or otherneeds. The aperture stop 33 can be designed to be very close to (i.e.,proximate to or just outside) the lens 24. In certain embodiments, afilter can be positioned in the aperture stop 33 to filter image lightpassing through, as will be appreciated by those skilled in the arthaving the benefit of the present disclosure.

Whether the clean-up element 34 (shown in FIGS. 2-3) included in theexemplary embodiment in FIGS. 1-3 is used may depend on desired imagecontrast. The clean-up element 34 can be sandwiched between two lenselements 24A, 24B that make up the lens element 24, as shown in FIG. 3,although other configurations are possible. The clean-up element 34could be cemented between the two elements 24A, 24B using a suitableoptical cement. In alternative embodiments, the clean-up element 34could be positioned at any appropriate location in the front group 14,for instance: between the reflecting linear polarizer 32 and the lenselement 24; between the lens elements 24 and 26; between the lenselements 26 and 28; or between the lens elements 30 and a display screen36 (see FIGS. 1-3). In this last position, the clean-up element 34 maybe attached (e.g., by suitable optical cement) to the lens element 30 orit may be completely external to the lens 12. The clean-up element 34 ispreferably positioned in the front group 14 at locations where the imagelight is not substantially diverging or of large ray angles.

The first lens unit 14 may include at least one aspherical surface orelement (i.e., an asphere). For example, in the exemplary embodimentshown in FIGS. 1-3, the lens elements 26 and 30 can be aspheres havingaspheric surfaces 26A and 30A, respectively, as shown in FIG. 3. Inother embodiments, different numbers of aspheric lens elements orsurfaces can be combined with non-aspheres, and exhibit analogous orsimilar performance characteristics to the projection lens system 10.Moreover, additional embodiments exhibiting analogous or similarperformance characteristics can include no aspheres and/or gradientindex or diffractive optical components, as will be appreciated by thoseskilled in the art having the benefit of the present disclosure. All ofthese embodiments are included within the scope and spirit of thepresent invention.

In the exemplary embodiment shown in FIGS. 1-3, the projection lenssystem 10 also includes imager 38 for color imaging and a chromaticseparator or beamsplitter 40. In general, as used herein, the imager 38is understood to mean one or more color imagers, for example, imagers38A, 38B, 38C for three-color imaging. Other numbers of imagers arepossible, for example, one, two, four, or more. The number of imagerswill depend, in general, on the specific implementation or design of theprojection lens system 10 and/or an illumination subsystem for theprojection lens 10. Examples of embodiments in which one or two imagerslike imagers 38A, 38B, 38C could be used are field sequential colorsystems, as will be appreciated by those skilled in the art having thebenefit of the present disclosure. For simplicity of presentation, insome of the drawings only one imager is shown, which is labeled as theimager 38 (see, e.g., FIGS. 2 and 3). In other drawings, all threeimagers 38A, 38B, 38C will be shown when the discussion warrants it orwhen easily drawn. In the view shown in FIG. 1, the imager 38C is notvisible as it is obscured by the chromatic separator 40. A separatecover glass 39A, 39B, 39C (indicated generally as numeral 39 in thedrawings showing only the imager 36) is included for each of therespective imagers 38A, 38B, 38C. There is a small (not shown) air gapbetween each cover glass 39A, 39B, 39C and the respective imagers 38A,38B, 38C. In other embodiments, the cover glass 39A, 39B, 39C may beintegrated with the imager 38A, 38B, 38C, there may be no air gap, orthere may be no cover glass at all. Each of the color imagers 38A, 38B,38C may be LCD imagers, such as ferroelectric LCD (FLCD) imagers, orother forms of imagers.

Any appropriate chromatic separator can be employed as the chromaticseparator 40. In FIGS. 1-3, the chromatic separator is shown simply as ablock. FIG. 4 offers a view of the imagers 38A, 38B, 38C and thechromatic separator 40 in an exemplary embodiment. The front lens unit14, the rear lens unit 16, and the reflecting linear polarizer 32 arenot in detail in FIG. 4. The chromatic separator in FIG. 4 is a Philipsprism, which is discussed further below. The chromatic separator 40splits the incoming white light received from an illumination subsystem(not shown in FIGS. 1-3) into three color bands, for example, the red,green, and blue primary colors, as generally indicated by respectivenumerals 42A, 42B, and 42C in FIGS. 2-4. The illumination subsystemincludes the light source and the incoming white light is received bythe chromatic separator 40 via reflection from the reflecting linearpolarizer 32, as discussed above. The incoming white light may besubstantially white or quasi-white light. Quasi-white light is definedto be light from a light source that is deficient in its output in oneor more colors (or wavelength bands) of the visible spectrum.Substantially white or quasi-white light will be referred to hereinsimply as white light. The chromatic separator 40 separates the primarycolors in the incoming white light in the exemplary embodiments shown inFIGS. 1-4. The color-separated light components 42A, 42B. 42C aredirected along different paths to corresponding ones of the imagers 38A,38B, 38C.

One way to direct the color-separated light 42A, 42B, 42C is to use thewell-known Philips prism as the chromatic separator 40, as alreadymentioned. The Philips prism is a type of chromatic separator thatincludes one or more prism elements, for example, prism elements 44A,44C, and an optional cover 44B, as shown in FIG. 4. Each of the prismelements 44A, 44C includes a highly reflective, multilayered coating(e.g., coatings 44D, 44E) designed to substantially reflect or transmitparticular colors of light to separate the colors. Each of the coatings44D, 44E preferentially reflect or transmit a color that is distinctfrom the colors reflected or transmitted by the multilayered coating onthe other prism element. In other words, the coating 44D is, in general,different, and reflects and transmits differently, than the coating 44E.In other embodiments, the chromatic separator 40 could take other formsthat function analogously or similarly to the Philips prism, such as thewell known X-cube beamsplitter.

In typical use, each of the three color imagers 38A, 38B, 38C receivesthe color-separated light or bands of light 42A, 42B, 42C derived fromthe illumination subsystem (i.e., from illumination light) and reflectsback a corresponding color-separated image imparted on each color band,as indicated schematically by numerals 46A, 46B, and 46C in FIGS. 2-4.The imagers 38A, 38B, 38C, if they are FLCDs, twisted nematic LCDs, orother types of spatial light modulators, each impart the respectivecolor-separated image under control derived from an external video orother control signal (not shown). The control signal can be implementedas a temporal electrical modulation of electrooptic states of individualpixels (not shown) that are defined in the imagers 38A, 38B, 38C. Eachpixel is individually electrically addressable for control of itsstates. One state (e.g., an “on” state) rotates (i.e., retards) thepolarization of incoming light by substantially 90 degrees. Retardationoccurs because the light impinging on the pixel makes a double passthrough a quarter-wave optical thickness of the pixel with anintervening reflection. A reflector located behind the pixel or forminga back part of the pixel provides the reflection. The other state (i.e.,an “off” state) does not substantially rotate the polarization before orafter reflection during the double pass. Projectable gray levels areachievable at intermediate states between the on and off states, forexample, if the imagers 38A, 38B, 38C are the twisted nematic LCDs,which have a variable birefringence with applied voltage. Intermediatevoltage values between the on and off state voltage values can produceanalog gray scale. The FLCDs are bi-stable devices and hence they wouldonly have the two states discussed (i.e., on and off).

At any instance in time during image formation, a particular electricalon and off state pixel pattern corresponds to the image information thatis imparted on the light 46A, 46B, 46C upon reflection from the imagers38A, 38B, 38C. This pattern is transformed into a pattern ofpolarization states of different bundles of the light 46A, 46B, 46C(i.e., into polarization-encoded bundles of the reflected light 46A,46B, 46C). The color-separated image information in the image light 46A,46B, 46C is then combined by the color separator 40. The bundles of thelight 46A, 46B, 46C traveling from the rear unit 16 toward the frontunit 14 are then selected according to their polarization state by thereflecting linear polarizer 32. Image light that had its polarizationrotated substantially by 90° by the imagers 38A, 38B, 38C issubstantially transmitted through the reflecting linear polarizer 32 aslight 48. Light (not shown) whose polarization was not substantiallyrotated is reflected by the reflecting linear polarizer 32 and out ofthe projection lens 12, back toward the illumination subsystem. Thereflected light travels essentially the same path in reverse of the paththat the incoming light took from the light source in the illuminationsubsystem. This reflected light could be used for optical pumping of thelight source for improved efficiency in the illumination subsystem, insimilarity to the discussion above.

The transmitted light 48 has substantially the second polarizationorthogonal to the previously desired (first) polarization of incominglight that was reflected by the reflecting linear polarizer 32 towardthe imagers 38A, 38B, 38C. The light 48, therefore, passes through thereflecting linear polarizer 32 and through the clean-up element 34, ifpresent. Characteristic directions of the clean-up element 34 and thereflecting linear polarizer 32 are aligned for this transmission, andthe clean-up element 34 selects the polarization further. The light 41then passes through the front lens unit 14 toward the screen 36 as imagelight 49, which forms a full color image projected thereon (see FIGS. 1and 2). The nominal throw of the projection lens 12 to the screen 36(i.e., the distance between them) is approximately 447 mm in air in theexemplary embodiments in FIGS. 1-4. Other embodiments can be designedwith different throw distances. The magnification to the screen 36 isapproximately 26, although other magnifications could be designed, aswill be appreciated by those skilled in the art having the benefit ofthe present disclosure. The magnification to the screen 36 isapproximately 26, although other magnifications could be designed, aswill be appreciated by those skilled in the art. With the use of opticaldesigns of different angular magnifications in the front and reargroups, the light can be imaged onto screens of different sizes. Forexample, the front lens unit can exhibit high angular magnification forwide field projection.

The projection lens 12 advantageously exploits the light output from theillumination subsystem that is imaged onto the imagers 38A, 38B, 38C bybeing telecentric or substantially telecentric in object space.Moreover, the numerical aperture (NA) of the projection lens 12 isnominally high. In the exemplary embodiments shown in FIGS. 1-4, theprojection lens 12 has an NA of approximately 0.1786, which isequivalent to an F# of approximately 2.8 in air. The projection lens 12could be designed to have other NA values.

Table 1 summarizes nominal projection lens 12 data for the exemplaryembodiments shown in FIGS. 1-4.

TABLE 1 GENERAL PROJECTION LENS DATA Operating Temperature (C.) 0°-60°C. Stop S18 [FIG. 4A and Table 2] Stop Diameter 16 mm Eff. Focal Length18 mm Object Space NA 0.1786 Image Diagonal 542 mm Magnification 26.25Entrance Pupil Position ∞ (telecentric) Exit Pupil Diameter 6.5 mm ExitPupil Position −473 from image Object Diagonal 20.6 mm Wavelength BandVisible Lens Units Millimeters

Table 2 is a summary of the projection lens 12 surface data for theexemplary embodiments shown in FIGS. 1-4. The columns in Table 2 are forsurface number, surface radius, thickness (i.e., distance between thesurface indicated in a row of Table 2 and the surface indicated in thenext row), glass/material (e.g., glass or other material or materialparameters), diameter, and conic (for aspheric surfaces). FIG. 4Aassociates the element surfaces in the second column of Table 2 with thenumerical elements in the first column of Table 2 and shown in FIGS.1-3. Surfaces S13-S16 represent interior surfaces of the exemplaryglass/optical cement/MOF/optical cement/glass embodiment of thereflecting linear polarizer 32 and are not specifically identified inFIG. 4A. Table 2 includes surfaces S19-S22 for the clean-up element 34,although the clean-up element 34 is optional.

Table 3 includes higher order aspheric coefficient entries for theaspheric surfaces S23 and S27 of the lens elements 26 and 30,respectively, decentering information for the surface S19, and tiltinformation for the surfaces S12 and S17.

FIG. 5 shows a projection lens system 50 in accordance with a secondembodiment of the invention. The system 50 is similar to the projectionlens system 10, and is a variation of the projection lens system 10. Aprojection lens 52 includes a front or first lens unit 14′, which issimilar to the lens unit 14 in the lens 12. The front lens unit 14′includes lens elements 24′, 26′, 28′, and 30′. An optional clean-upelement 34′ can be sandwiched between lens elements 24A′ and 24B′ ofwhich the lens element 24′ is constructed. The front lens unit 14′ alsoincludes a remote aperture stop 33′. The elements 24′ (24A′ and 24B′26′, 28′, 30′, 33′, and 34′ are analogous or similar to the elements 24(24A and 24B), 26, 28, 30, 33, and 34, respectively, in the projectionlens 12. The system 50 further includes the imager 38 and the chromaticseparator 40.

TABLE 2 PROJECTIONS LENS SURFACE DATA SUMMARY Dwg. Element No. Surf No.Radius Thickness Glass/Material Diameter Conic 38 OBJECT AT Infinity 1.1ZKN7 22.4 IMAGER S1 Infinity 0.8 22.4 40 S2 Infinity 40 BK7 36.72 S3Infinity 1.5 36.72 18 S4 Infinity 2.5 SF11 37.6 S5  58.57127 8.8 SK537.6 S6  −44.27827 0.5 37.6 20 S7  102.6493 9.4 SK5 37.6 S8  −34.326822.5 SF11 37.6 S9 −101.7558 0.5 37.6 22 S10  45.26133 5.1 BK7 36 S11 142.304 17.704 36 32 S12 — 0 — S13 Infinity 0.7 BK7 37.5 S14 Infinity0.125 index 1.580000 37.1 Abbe number 58.000 S15 Infinity 0.7 BK7 37 S16Infinity 0 36.6 S17 — 12.596 33 Stop S18 — 0 16 24, S19  27.40444 4BASF2 20 34 (S20, S20 Infinity 0.125 index 1.450000 20 S21) Abbe number58.000 S21 Infinity 2.65 BASF2 20 S22  51.24619 19.2 20 26 S23  30.248374.5 ACRYLIC 29.4 0.41940 S24  33.3 10.5 29.4 28 S25  −14.45517 6.8 BK724.8 S26  −35.82717 1.55 37.6 30 S27  −26.2 4.3 ACRYLIC 45 −1.5309  S28 −66 447 45 36 SCREEN Infinity — 577.872 IMAGE S29

TABLE 2 PROJECTIONS LENS SURFACE DATA SUMMARY Dwg. Element No. Surf No.Radius Thickness Glass/Material Diameter Conic 38 OBJECT AT Infinity 1.1ZKN7 22.4 IMAGER S1 Infinity 0.8 22.4 40 S2 Infinity 40 BK7 36.72 S3Infinity 1.5 36.72 18 S4 Infinity 2.5 SF11 37.6 S5  58.57127 8.8 SK537.6 S6  −44.27827 0.5 37.6 20 S7  102.6493 9.4 SK5 37.6 S8  −34.326822.5 SF11 37.6 S9 −101.7558 0.5 37.6 22 S10  45.26133 5.1 BK7 36 S11 142.304 17.704 36 32 S12 — 0 — S13 Infinity 0.7 BK7 37.5 S14 Infinity0.125 index 1.580000 37.1 Abbe number 58.000 S15 Infinity 0.7 BK7 37 S16Infinity 0 36.6 S17 — 12.596 33 Stop S18 — 0 16 24, S19  27.40444 4BASF2 20 34 (S20, S20 Infinity 0.125 index 1.450000 20 S21) Abbe number58.000 S21 Infinity 2.65 BASF2 20 S22  51.24619 19.2 20 26 S23  30.248374.5 ACRYLIC 29.4 0.41940 S24  33.3 10.5 29.4 28 S25  −14.45517 6.8 BK724.8 S26  −35.82717 1.55 37.6 30 S27  −26.2 4.3 ACRYLIC 45 −1.5309  S28 −66 447 45 36 SCREEN Infinity — 577.872 IMAGE S29

The front lens unit 14′ is laterally adjustable as a group with respectto the remainder of the projection lens 50. Lateral adjustment can bemade by decentering along X and Y axes in a right-handed coordinatesystem 54 shown in FIG. 5. The direction of motion is also generallyindicated by the double-headed arrow 56 parallel to the Y axis and theorthogonal arrowhead/tail 58 parallel to the X axis.

The purpose of decentration is to mitigate possible effects ofmanufacturing tolerances within the projection lens 50 to improve imagequality. The mechanism for decentration in the embodiment shown in FIG.5 could be implemented in various configurations, as will be appreciatedby those skilled in the art having the benefit of the presentdisclosure. One exemplary mechanism is shown in FIGS. 6A and 6B inaccordance with an embodiment of the invention. FIGS. 6A and 6B show aportion of a housing 60 of the front lens unit 14′ of the projectionlens 52. In this embodiment, the front lens unit 14′ is constructed as amodular barrel 62 that installs into the housing 60 in a directiongenerally indicated by arrow 64. When the front lens unit 14′ is fullyinserted into the housing 60 (FIG. 5B), flat 66 rests on flat 68. Thehousing 60 can be articulated along axes 70 and 72 by suitableadjustment known in the art (e.g., by screw adjustment). In oneexemplary embodiment, the barrel 62 is manipulated with an externaldevice (not shown), such as a screwdriver, until the opticalcharacteristics of the projection lens 52 are measured for bestperformance. The barrel 62 is then glued in place with an appropriateglue.

In accordance with an embodiment of the invention, an illuminationsubsystem includes an illumination relay lens system for introducinglight from a light source to the projection lenses 12, 52. One exemplaryembodiment including such an illumination subsystem 74 is illustrated inFIG. 7. An illumination relay lens system 76 receives light from a lightsource 78A. The illumination relay lens system 76 directs light outputfrom the light source 78A to the reflecting linear polarizer 32 in theprojection lenses 12, 52. The rear lens unit 16 in the projection lenses12, 52 is common to light paths of the illumination subsystem 74 (orother types of illumination subsystems discussed herein) and theprojection lens systems 10, 50. The magnification of the illuminationrelay lens system 76 is approximately two in one embodiment. In otherembodiments, the illumination relay lens system 76 may include one ormore aspheres (e.g., constructed of a polymer, such as acrylic), and mayhave different magnifications and element powers.

In FIG. 7, the light source 78A includes a lamp 80A and a lamp powerdrive or power source (not shown). The lamp 80A may be driven byelectric arc, radiofrequency (rf) energy, microwave, or like powersource and include equipment or hardware (not shown) for coupling powerto the light emitting material of the lamp 80A. The lamp 80A can be oneof the lamps described in the aforementioned U.S. patent applicationSer. Nos. 08/747,190 or 08/771,326, or in U.S. Pat. No. 5,404,076,entitled “Lamp Including Sulfur,” and U.S. Pat. No. 5,606,220, entitled“Visible Lamp Including Selenium or Sulfur,” both issued to Dolan etal., which are incorporated by reference herein in their entirety.

FIG. 8 shows a projection system and an illumination subsystem inaccordance with a third embodiment of the invention. A light source 78Bis similar to the light source 78A and includes a lamp 80B (similar tothe lamp 80A) and a lightpipe (e.g., a tapered lightpipe or TLP) 82,which is a type of lightguide. The lamps 80A, 80B and the TLP 82 will bediscussed further below. Like Table 1, Table 4 summarizes generalprojection lens 12, 52 data and illumination relay lens system 74 datafor the embodiment shown in FIG. 8. Table 5 is a summary of theprojection lens 12 (and 52) surface data for FIG. 8, in similarity toTable 2.

FIG. 8A associates the element surfaces in the second column of Table 5with the numerical elements in the first column of Table 5 and shown inFIG. 8. Surfaces S13-S18 represent interior surfaces of the exemplaryembodiment of the reflecting linear polarizer 32 and are notspecifically identified in FIG. 8A, as similarly discussed above forFIG. 4A. No meaning should be attached to the use of similar elementsurface numerical labels between the embodiments shown in Table 2 (andFIG. 4A) and Table 5 (and FIG. 8A).

TABLE 4 GENERAL LENS DATA No. Surfaces 30 Temperature (C.) 0°-68° C.Object Space N.A. 0.32 Eff. Focal Length −172 mm Working F/# 2.65 StopDiameter 16 mm Paraxial Magnification −1.78 Object Height in Millimeters12.5 mm diagonal Primary Wavelength 0.556 microns Lens Units Millimeters

The illumination relay lens system 76 is designed to accommodate theextent or size of the light output from the TLP 82. In a particularembodiment, the TLP 82 and the imager 36 are not substantiallyadjustable relative to each other while their adjustment can be made inother embodiments or in other ways in still other embodiments. Forexample, an illumination field stop 83 (see FIGS. 7, 7A, and 8) can belaterally adjusted to allow light passing from the light sources 78A,78B to the imager 38 to be centered on the imager 38. The field stop 83can be a rectangular field stop.

In the exemplary embodiment shown in FIG. 8, a pre-polarizer 86 is alsoincluded in an aperture stop 84. The pre-polarizer 86 can be amulti-layered or sandwiched structure in a heat-sink frame, such aslayers of DBEF (or MOF), glass, air, sapphire, and an absorptionpolarizer (e.g., with optical cement in between each adjacent layer).The sapphire acts as a heat collector and the pre-polarizer 86 can beAR-coated. With this construction, the sapphire layer may be usedadvantageously as a heat sink, depending on the design of the lightsource 78A, 78B. The MOF layer of the pre-polarizer 86 may be used toreflect light of an undesired polarization (i.e., polarization notaligned for reflection to the imager 38 by the reflecting linearpolarizer 32) back to the lamp 80B for optical pumping, as discussedabove, as well as to limit the amount of light absorbed by theabsorption polarizer to minimize heating effects. On the other hand, theMOF layer transmits light of the desired polarization (i.e.,polarization aligned for reflection to the imager 38 by the reflectinglinear polarizer 32).

The illumination relay lens system 76 may also include an IR/UV filteror coating 88 on a lens 90. Infrared filtering can reduce orsubstantially mitigate detrimental thermal effects from high poweredlamps in imaging systems. Ultraviolet filtering can reduce orsubstantially mitigate degradation of optical bonding materials (e.g.,optical cements or epoxies) if they are used in the projection lenssystems 10, 50. The IR/UV filter 88 shown in FIG. 8 reflectsnear-visible IR radiation from the light source 78A, 78B away from theprojection lenses 12, 52 and back toward the lamp 80B. The IR radiationabove approximately 1.2 microns is absorbed by the absorption polarizerin the IR/UV filter 88. In alternative embodiments, only an IR or a UVfilter (i.e., not both), or no filter, may be used. The IR/UV filter 88(or separate IR or UV filters) could be in other positions, as will beappreciated by those skilled in the art having the benefit of thepresent disclosure. For example, the filter 88 could be a UV filter or aUV coating on the lens 90 and an IR mirror could be placed near or inthe middle of the illumination relay lens system 76 between the lenses74A and 74B in FIGS. 7, 8 and 8A. With an IR (hot) mirror, UV could beabsorbed and so there may be no need for a separate UV filter orcoating.

TABLE 5 ILLUMINATION RELAY SYSTEM SURFACE DATA SUMMARY Dwg. Element No.Surface No. Radius Thickness Glass/Material Diameter Conic e.g., 82OBJECT (e.g., Infinity  0.005 12.5 0 TLP OUTPUT) 90 S1 Infinity  2.3SILICA 16 0 90, 90A S2  −25.2  2.877403 16 0 76 S3 Infinity  4.5 BK7 200 S4  −18.59391  14.45 20 0 S5 Infinity  5.5 BK7 22 0 S6  −27.77859  0.522 0 S7 Infinity  3.7 BK7 22 0 S8  −34.7788  2.22 22 0 86 S9 Infinity 1.5 BK7 18 0 S10 Infinity  0 18 0 84 STOP 11 Infinity  14.35 — 16 — 32S12 —  0 — — — S13 Infinity  0.7 BK7 36.52763 0 S14 Infinity  0 MIRROR37.41535 0 S15 Infinity  −0.7 BK7 37.41535 0 S16 Infinity  0 38.30535 0S17 —  0 — — — S18 Infinity −18.225 23.73374 0 S19 —  0 — — — 22 S20 142.304  −5.1 BK7 37 0 S21  45.26  −0.5 37 0 20 S22 −101.755  −2.5 SF1137.5 0 S23  −34.326  −9.4 SK5 37.5 0 S24  102.649  −0.5 37.5 0 18 S25 −44.278  −8.8 SK5 37.5 0 S26  58.571  −2.5 SF11 37.5 0 S27 Infinity −1.5 37.5 0 40 S28 Infinity  −40 BK7 36.72 0 S29 Infinity  −0.8 36.72 039 S30 Infinity  −1.1 ZKN7 24.75913 0 38 IMAGE AT Infinity  — 22.93756 0IMAGER S31 Surface S19 Y decentered −0.52 mm Surfaces S12 & S19 tilted45°

The TLP 82 is four-sided, pyramidal-shaped, and rectangular incross-section, with flat sides and ends in the exemplary embodimentshown in FIG. 8. The TLP 82, having this structure, is used to“condition” the light, although other shapes could be used. The TLP 82accepts light from the lamp 80B and guides the light substantially bytotal internal reflection (TIR), as will be appreciated by those skilledin the art having the benefit of the present disclosure. The lightreceived from the lamp 80B is multiply reflected within the TLP 82 as itundergoes TIR and is output by the TLP 82 to the illumination relay lenssystem 76. The TLP 82 exhibits TIR because of its shape and its opticaland material properties, and because of its orientation for receivinglight from the lamp 80B. In FIG. 8, the TLP 82 is shown. bonded to,layered with, or otherwise attached to a lens element 90 (e.g., apositive lens). The lens 90 can also be integral with the TLP 82 inother embodiments. In the exemplary embodiment shown in FIG. 8, the lenselement 90 includes a lens surface 90A bonded to the TLP 82 with theUV/IR filter coating 88 in between. It will be appreciated by thoseskilled in the art having the benefit of the present disclosure that theIR/UV filter or coating 88 could be disposed at other positions withinthe system 76 or on the surface of the lens element 90 away from the TLP82. Embodiments, for example, as shown in FIGS. 9 and 10, in which thelens surface 90A is integral with the TLP 82 are simple, low cost, andradiometrically efficient.

In operation, the relay system 76 images the light output from the TLP82 onto the imager 38. Light is both homogenized and controlled in solidangle by the TLP 82 to allow for simple imaging onto the imager 38 withlittle loss. The TLP 82 conditions the light output from the lamp 80B tobecome substantially telecentric light at the imager 38. The light fromthe TLP 82 is provided at the right NA to the illumination relay lenssystem 76 to produce the near-telecentric light at the imager 38. Inalternative embodiments, a condenser, which is also a light homogenizer,could be used instead of the TLP 82 and the relay system 76. Thecondenser would form an image of the light source at the entrance pupilof the projection lens, thereby matching the illumination system to theprojection system.

FIG. 11 illustrates an exemplary mounting apparatus for holding the TLP82 in accordance with an embodiment of the invention. In FIG. 11, thelens 90 is bonded to the TLP 82 with the IR/UV coating 88 in between.Bonding is made using a suitable optical adhesive. The lens 90 ismounted in the illumination subsystem 74 by mount 92, shown incross-section in FIG. 11, which can completely encircle the lens 90along an edge 94 of the lens 90. The lens 90 could be glued ormechanically retained within the mount 92. The mount 92 is completelyoutside the light cone 96 passing through and out of the TLP 82. Theapparatus shown in FIG. 11 is a desirable embodiment, because light lossdue to loss of TIR can be reduced or avoided if the TLP 82 were notmounted and, therefore, not contacted on its side 98, or on its end 100.A physical mount 102 at the end 100 is optional. The mount 102 cancompletely or substantially decouple the TLP 82 from the lamp 80B, whichmay afford prevention or reduction of possible physical and thermaldegradation.

FIGS. 12, 13, and 14 show some variations in ways to mount the TLP 82 inaccordance with alternative embodiments of the invention. FIG. 12illustrates a mount 106 similar to the mount 92 (e.g., it is completelyoutside the light cone 90 passing through and out of the TLP 82).Physical contact is made between the lamp 80B and/or its housing 108 andthe TLP 82. The TLP 82, the mount 106, and the lens 90 are positioned sothere is, in general, a force directed toward the lens 90 from the lamp80B. No adhesive is required between the lens 90 and the TLP 82 orbetween the lens 90 and the mount 106. The force pushes the TLP 82against the lens 90 in the mount 106. FIG. 13 shows a detail of thecontact made between the TLP 82 and the lens 90. FIG. 14 shows anotherdetail with the TLP 82 including a ground corner region 109 thatsubstantially conforms to the curvature of the lens 90. The exemplaryembodiment shown in FIG. 14 may provide improved fragility in the cornerregion 109, both for the TLP 82 and the lens 90. The UV/IR coating 88(or only one of them, as discussed above), although not shown in FIGS.12 or 13, can be disposed in between the TLP 82 and the lens 90.

In an illumination subsystem 110 shown in FIGS. 15A and 15B, inaccordance with another alternative embodiment of the invention, a lightfunnel 112A/compound parabolic concentrator (CPC) 112B in a combination112, having reflective inner surfaces 114 and 115, or other type orshaped lightguide may be employed instead of the TLP 82 in a relaysystem. A lightpipe (e.g., a non-tapered light homogenizer) 113 isincluded in the subsystem 110 to homogenize the light received from alight source (e.g., 80A or 80B) that passes through the combination 112on its way to an illumination relay lens system (e.g., the system 76).The light funnel portion of the combination 112 is a funnel-shaped,reflecting optical element. Devices similar to the CPC 112B and the TLP82 are described in U.S. Pat. Nos. 5,237,641, 5,243,459, 5,303,322,5,528,720, 5,557,478, 5,610,768, and 5,594,830, which are incorporatedby reference herein in their entirety. The region 117 between the funnel112A and the CPC 112B is a region of high light energy. The cone angle θ(see FIGS. 15A and 15B) of the funnel 112A, which determines the coneangle of the light through the lightpipe 113, is preserved at the outputof the lightpipe 113, as shown in FIG. 15B. The light output from theCPC 112B is telecentric or substantially telecentric. The angle θdetermines the angle of the cone of light at the output of theillumination relay lens system, and is related to its telecentricity. Incertain alternative embodiments, the funnel 112A is not included withthe CPC 112B. In still other alternative embodiments, the TLP 82 can bereplaced with a system of lenses that may include one or more asphericsurfaces and/or gradient index or diffractive optics that freely imagelight from the lamp 80B to the imager 38. Such alternative embodimentsalso provide well behaved, substantially telecentric cones of light tothe imager 38. The TLP 82, as well as the illumination relay lens system76, could also both be replaced by a completely different illuminationrelay lens system of another design, as will be appreciated by thoseskilled in the art having the benefit of the present disclosure. All ofthese embodiments image light from the lamp 80B onto the imager 38,providing substantially telecentric and uniform light. Moreover, the TLP82, as well as these other types of lightguides and relay lens systems,beneficially allows for the use of an arc lamp, such as a metal halidelamp, or other lamp types. They also provide high efficiency for highillumination brightness and uniformity.

As mentioned above, in accordance with embodiments of the invention, theillumination subsystem 74 and the imager 38 light can be adjustedrelative to each other. In one embodiment, the position of the TLP 82can be adjusted by simple mechanical adjustment (e.g., by a screwadjustment) relative to the illumination relay lens system 76. Forexample, the TLP 82 can be laterally or angularly adjusted relative tothe system 76, which controls the cones of the light impinging on theimager 38 to also move laterally or angularly, as will be appreciated bythose skilled in the art having the benefit of the present disclosure.

In another embodiment shown in FIGS. 16 and 17, the reflecting linearpolarizer 32 can be adjusted about one or more axes of rotation toadjust the illumination subsystem 74 (and hence, the TLP 82) and theimager 38 relative to each other. In this embodiment, the adjustablefield stop 83 is not needed to adjust the illumination on the imager 38,and is not necessarily present. This will allow the substantiallytelecentric light received from the illumination subsystem 74 via theillumination relay lens system 76 to be adjusted on the pixel faces ofthe imager 38. Adjustment of the polarizer 32 can be used to optimizethe coupling of light between the output of the TLP 82 and the imager38. Moreover, as with adjustment of the front lens unit 14′ (see FIG.5), adjustment of the polarizer 32 can be used to compensate formanufacturing or mounting tolerances.

In more detail, FIGS. 16 and 17 show a beamsplitter adjustment device130 that includes adjustment screws 132A, 132B, adjustment cams 134A,134B (e.g., 2:1 cams), an adjuster 136, and a pivot 138 (not shown inFIG. 17). The adjustment device 130 can be constructed of molded plasticcomponents attached to the inside of the projection lenses 12, 52 (notshown in detail in FIG. 16). The adjustment screws 132A, 132B contactthe cams 134A, 134B, respectively. The cams 134A, 134B rotate againstthe adjuster 136 to rotate or tilt the polarizer 32. Turning both of theadjustment screws 132A, 132B causes a top 32A of the polarizer 32 totilt up to approximately 1° from a vertical plane as generally indicatedby arrow 140. Turning only the adjustment screw 132A causes thepolarizer 32 to rotate up to approximately 1° from a 45° plane, asgenerally indicated by arrow 142. The adjuster 136 also returns the cams134A, 134B in the corner (i.e., the adjuster 136 holds the cams 134A,134B in place inside the projection lenses 12, 52). Springs 144A, 144Bbias the adjustment screws 132A, 132B against the cams 134A, 134B. Lightreceived by the polarizer 32 can be steered to the imager 38 byadjustment of the adjustment screws 132A, 132B to optimally illuminatethe imager 38. The rear lens unit 16 is located in the directionindicated by arrow 146 in FIGS. 16 and 17.

Other modifications besides mechanical adjustment mechanisms foroptimizing illumination of the imager 38 can be made to the projectionlenses 12, 52 based on other considerations. For example, the lenses 12,52 and other nominal lens designs are multi-color projection lenses,which may exhibit residual color fringing, also referred to as lateralcolor. Color fringing is a result of a higher or a lower magnificationfor one or more colors compared to the other colors in the opticalsystem. For example, red and blue light may image at higher or lowermagnification than green light. For light exhibiting lower red and bluemagnification, adding a very weak, negatively powered element near thegreen imager (e.g., 38A, 38B, or 38C) in the green light path or channelcan decrease the magnification of the green light to compensate for themagnification differences. Thus, the red, green, and blue light can besubstantially and simultaneously matched in magnification to the othercolors. Other similar or analogous embodiments for correcting colorfringing also use one or more weak lens elements for the red and/or theblue channel in addition to the green channel, or instead of the greenchannel. More than one color and/or other colors besides red, green, orblue may be corrected. These embodiments and other embodiments that useweak, positive lenses or combinations of weak negative and positive lenselements to decrease or increase the magnification in one or morechannels to correct color fringing are included within the scope andspirit of the present invention.

A system for correcting color fringing in one color channel (e.g., thegreen channel) is shown in FIG. 18 and in FIG. 18A in more detail inaccordance with a fourth embodiment of the invention. A projection lenssystem 150, which is a variation of the projection lens systems 10 and50, includes a projection lens 152 (shown schematically as a cut-awayblock in FIG. 18). The system 150 may be similar to the systems 10, 50(e.g., including the adjustment mechanisms described above), except forthe addition of a weak correcting element 154 (e.g., a lens). The weakcorrecting element 154 is disposed between the color separator 40 (alsosee FIG. 1-3) and the one imager of the imagers 38A, 38B, or 38C (inthis case imager 38B is illustrated) that is being used to impart theparticular color image on the incoming light that needs to be correctedfor lateral color. The element 154 can be bonded to (e.g., with anappropriate optical cement) either the chromatic separator 40 or to thecover glass associated with that particular imager. Alternatively, theelement 154 can be disposed and held in place between the chromaticseparator 40 and the imager by an appropriate mount. The correctingelement 154 includes at least one curved surface 154A, which exhibitsoptical power.

The weak correcting element 154 is used to bring the color to becorrected (e.g., green) into substantial coincidence with the othercolors (e.g., red and blue light) upon recombination of the light in thecolor separator 40 on its way to the front lens units 14, 14′. This isillustrated in FIG. 18 by considering the following. Pixel A on thegreen imager 38B corresponds to pixel C on the blue imager 38C and topixel D on the red imager 38A. The projection lens 152 has the lateralchromatic aberration, which is the change in magnification for differentcolors. The green channel has a larger magnification than the red andthe blue channels. To correct this difference in magnification in theprojection lens 152, the weak negative lens 154 has been added in frontof the green imager 38B. The light 46B coming from the pixel A, which isthe correct light, is redirected by the lens 154 such that it appears asif it is coming from the pixel B, which is closer to the center of theimager 38B. In other embodiments, gradient index, aspheric lenses, ordiffractive optics could be used for the weak correcting element 154.For example, diffractive optics in the channel for the color to becorrected in the projection lens could be used (e.g., in the rear unit)instead to correct color fringing. Another way to accomplish this is touse a diffractive optical element common for all three color channels.

In the exemplary embodiment shown in FIG. 18, the weak correctingelement 154 is used to reduce or eliminate color fringing in the greenchannel, which uses the imager 38B having the cover glass 39B (althoughinclusion thereof is dependent on the particular of the design imager38B). The other colors have corresponding color images imparted thereonin the projection lens 152 by operation of the imagers 38A, 38C. Inother embodiments, weak correcting elements may be employed to correctthe colors associated with the other imagers (i.e., 38A and 38C).

Another consideration for the projection systems 10, 50, 150 is the useof high power light sources, such as those described in theaforementioned U.S. Pat. Nos. 5,404,026 and 5,606,220 and in U.S. patentapplication Ser. Nos. 08/747,190 and 08/771,326, when aspheric elementsor aspheres are also included in the lenses 12, 52, 152. Aspheres arefrequently constructed of a polymer or polymer materials. Certainpolymer materials, although exhibiting excellent optical properties, canalso exhibit detrimental thermal effects due to temperature changes thatoccur in the materials when high power light passes through the polymerasphere. High power light can negatively impact the projected imagesthrough these temperature changes. For example, aspheres constructed ofacrylic material are subject to changes in refractive index withincreasing temperature. This is because of a high coefficient of thermalrefractive index change. As a result, focus can change with temperature.Clever design using aspheres, however, may enable this thermal effect tobe substantially canceled or eliminated, which is termedathermalization. The projection systems 10, 50, 150 shown in FIGS. 1-3,5 and 18 implement athermalization. The projection lenses 12, 52, 152provide athermalization by carefully designing them to use aspheres(e.g., the lens elements 26 and 30) and to shift (e.g., positive)optical power from the aspheres (which can, therefore, have weak opticalpower) to the other elements that pass the high power light earlier asthe light proceeds through the front lens units 14, 14′. These otherelements are the lens elements 18, 20, 22, or 24, which allow theaspheric element 26 to be designed with lower (e.g., positive) opticalpower than might otherwise be required. These other elements can beconstructed of glass, which is less subject to thermal refractive indexchanges than are the polymer aspheres. Related detrimental thermalimaging effects are thereby avoided or prevented. The same thermalproblem is unlikely to occur with negatively powered lens elements, suchas the aspheric element 30, where the beam diameter is small for anyfield position. If the aspheres were made of glass instead of polymer,such thermal effects could likewise be reduced or eliminated. In thislatter case, the glass aspheres would not necessarily be limited tohaving weak optical power.

The remote aperture stop projection lenses 12, 52, and 152 describedherein offer improved optical performance. The benefits of having aremote aperture stop include better exclusion of out-of-angle light thanconventional lens designs. The remote aperture stop projection lenses12, 52, 152 also provide wide fields of view, are telecentric, andexhibit excellent resolution and near zero distortion. The projectionlenses 12, 52, 152 are compact and manufacturable. They also minimizeghost image formation and offer improved uniformity of screenbrightness. For athermalization purposes, strategic use can be made oftwo (or more) aspheric surfaces (e.g., constructed of acrylic material)in these compound lenses that are otherwise composed substantially ofspherical glass surfaces and materials. Moreover, the projection lens152 can additionally provide substantial lateral color correction forcolor imaging with light passing through the same optical components(aspheric glass elements are also feasible).

The projection lens systems 10, 50, 150 may be similar to image enginesdescribed in prior, co-owned U.S. patent application Ser. No.08/730,818, filed Oct. 17, 1996, by Richard M. Knox, entitled “ImageProjection System Engine Assembly,” which is incorporated by referenceherein in its entirety. The projection lens systems 10, 50, 150 may beadvantageously employed in front or rear projection systems, such as“folded” or “folded optics” display apparatuses. The display apparatuses200 and 250 shown in FIGS. 19 and 20, respectively, are examples ofthese folded apparatuses in accordance with fifth and sixth embodimentsof the invention. One or more imager configurations (e.g., that use twoor three imagers, like the imagers 38A, 38B, 38C) using color liquidcrystal filters may be employed. The display apparatuses 200 and 250 canbe part of a computer monitor or television display. They are similar tothe projection systems described in prior, co-owned U.S. patentapplication Ser. No. 08/581,108, filed Dec. 29, 1995, by Richard M.Knox, and in European Pat. app. No. 96309443.8, EP0783133A1, filed Dec.23, 1996, by Richard M. Knox et al., published Jul. 9, 1997, bothentitled “Projecting Images,” which are incorporated by reference hereinin their entirety. Such a “double bounce” geometry offers distinctadvantages. For instance, the folded optical paths in the displayapparatuses 200, 250 enable the size of the apparatuses 200, 250 to bereduced compared to other types of display apparatuses. This isillustrated in FIGS. 19 and 20, where the “footprint” dimensions “L” and“L′,” respectively, may be made smaller by folding the optical paths,making the apparent or effective projection lengths seem longer than theactual projection lengths.

Referring to FIG. 19, the display apparatus 200 includes an image engineor projector 202. The image engine 202 may be similar to the projectionlens systems 10, 50, 150. The image engine 202 may also be similar tothe image engines described in the aforementioned U.S. patentapplication Ser. No. 08/730,818. The image engine 202 outputs imagelight 204 in response to input signals, for example, electronic, video,or other signals received from an antenna, cable, computer, orcontroller. The image light 204 (e.g., the image light 49 from theprojection lenses 12, 52 in FIGS. 1-4 or analogous image light from theprojection lens 152) reflects off a lower mirror or reflector 206 to ahigher mirror or reflector 208. The light 204 is then reflected by theupper mirror or reflector 208 and is directed to a screen 212, forexample, a diffusive screen or diffuser. The screen 212 (e.g., similarto the screen 36) scatters the image light as light 214, which a viewer215 can see as forming an image at the screen 212 of the displayapparatus 200.

Referring to FIG. 20, the display apparatus 250 is shown, which includesan image engine or projector 252, a signal splitter 254, an input cable256, a sound system 258, a screen apparatus 260, and a back mirror orreflector 262. The image engine 252 may be similar to the projectionlens systems 10, 50, 150 described above and those in the aforementionedU.S. patent application Ser. No. 08/730,818. The screen apparatus 260includes a reflecting linear polarizer 264 and a screen 268, which,depending on the specific design, may be layered, coated, bonded (e.g.,with index matching adhesive), laminated (e.g., as one element), orotherwise applied together in the order shown in FIG. 20. The reflectinglinear polarizer 264 and the screen 268 may be held together with no airgap or with substantially no air gap. Alternatively, in otherembodiments, the reflecting linear polarizer 264 and the screen 268 maybe held together in spaced apart relation.

The screen 268 (e.g., similar to the screen 36) may be a diffusivescreen or a diffuser, and the reflecting linear polarizer 264 may beconstructed of MOF. Other polarizing reflector or wide-angle polarizingreflector materials could also be used. The reflecting linear polarizer264 has the characteristic of preferentially reflecting light of onelinear polarization and preferentially transmitting light of another,linear but orthogonal, polarization, as discussed above.

The back reflector 262 includes a mirror or reflector 270 and anachromatic retarder 272 that, depending on the design, may be layered,coated, bonded (e.g., with index matching adhesive), adjacent orotherwise applied together in the order shown in FIG. 20. The backmirror or reflector 270 and the achromatic retarder 272 may be heldtogether in spaced apart relation or not be held spaced apart (i.e.,with substantially no air gaps). A suitable achromatic retarder 272 maybe designed to accommodate the spaced apart arrangement, as will beappreciated by those skilled in the art having the benefit of thepresent disclosure.

In operating the display apparatus 250, the image engine 252 receives anelectronic signal through the input cable 256 and provides the signal tothe signal splitter 254. The signal splitter 254 divides the signalinto, for example, a video signal and an audio signal, and providesthese signals to the image engine 252 and the sound system 258,respectively. The image engine 252 converts the video signal intoprojected image light 274 (e.g., the image light 49). The electronicsignal received by the cable 256 may be any type of signal containingvideo information, such as a television signal received by an antenna orover cable lines, or a computer video signal received through a computervideo cable. The audio signal and the sound system are optional.

The image light 274 may be polarized in the image engine 252, forexample, by the operation of the reflecting linear polarizer 32, theimagers 38A, 38B, 38C, the pre-polarizer 86, and the clean-up element34, if present, as described above. A light source (not shown) in theimage engine 252 or other light source may be used to input linearlypolarized light initially into the image engine 252 in an illuminationsubsystem similar to those described above. The light would then beprocessed by the polarizer 32, the imagers 38A, 38B, 38C, and thepre-polarizer 86, the clean-up polarizer, if present, or as determinedby an external polarizer. The image light 274 may be polarized in thesecond polarization discussed above or have its polarization determinedby another polarizer that is employed external to the projection lens(not shown in FIG. 20) of the image engine 252. In a first instance, theimage light 274 output from the image engine 252 is polarized in thesecond polarization direction, for example. The light 274 is thenreflected by the reflecting linear polarizer 264 toward the backreflector 262. The reflected image light 274 passes through theachromatic retarder 272 a first time in one direction, is reflected bythe back mirror or reflector 270, and passes through the achromaticretarder 272 a second time, directed again toward the screen apparatus260. The achromatic retarder 272 is designed to have an opticalthickness of substantially one-quarter wave, such that the image light274 in the second polarization will undergo an effective half-wave(i.e., substantially 90 degrees) polarization shift or rotation ondouble pass through the achromatic retarder 272. Thus, the image light274, which is now directed toward the screen apparatus 260, willsubstantially be in the first polarization and will substantially passthrough the reflecting linear polarizer 264 and to the screen 268. Thescreen 268 scatters this light as image light 276. The viewer 215 canthen observe an image produced by the image light 276 at the screen 268of the screen apparatus 260, in similarity to the descriptions givenabove.

In all embodiments of the invention, diffusive viewing screens or beadedscreens may be used as the screens 36, 212, and 268. Beaded screenscapture stray imaging light, have a limited acceptance angle, and thestray light is absorbed in a black matrix. Diffusive screens, on theother hand, scatter the stray light to improve homogeneity and/oruniformity in intensity across the viewing screen. The type of diffusivescreens discussed herein include bulk diffusive screens. Surfacediffusers, for example, ground glass and the like, could also be usedinstead of diffusive screens or beaded screens in accordance with otherembodiments of the invention.

The particular embodiments disclosed above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown, other than as describedin the claims below. It is therefore evident that the particularembodiments disclosed above may be altered or modified and all suchvariations are considered within the scope and spirit of the invention.Accordingly, the protection sought herein is as set forth in the claimsbelow.

What is claimed is:
 1. A projection lens apparatus, comprising: a frontlens unit; a back lens unit optically coupled to said front lens unit; alinear reflecting polarizer optically coupled to the front lens unit andthe back lens unit that receives illumination light and directs theillumination light to the back lens unit; a chromatic separating elementoptically coupled to the linear reflecting polarizer that separates theillumination light into color separated light; an imager opticallycoupled to the chromatic separating element and adapted to impart animage on a portion of the color separated light; and a field lensadapted to change the magnification of the portion of the colorseparated light to substantially correct a chromatic aberration.
 2. Theprojection lens apparatus of claim 1, wherein the chromatic separatingelement is adapted to mix the image light with other image light.
 3. Theprojection lens apparatus of claim 1, wherein the field lens is adjacentto the chromatic separating element.
 4. The projection lens apparatus ofclaim 1, wherein the field lens is adjacent to the imager.
 5. Theprojection lens apparatus of claim 1, wherein the field lens is adaptedto substantially correct lateral color.
 6. The projection lens apparatusof claim 1, wherein the field lens is adapted to adjust magnification ofthe portion of the color separated light relative to the magnificationsof other portions of the color separated light.
 7. The projection lensapparatus of claim 1, wherein the field lens is adapted to compensatefor chromatic separating characteristics of the chromatic separatingelement.
 8. The projection lens apparatus of claim 1, further comprisinganother imager adapted to direct other portions of the color separatedlight as other image light.
 9. The projection lens apparatus of claim 8,wherein the chromatic separating element is adapted to mix the imagelight and the other image light.
 10. The projection lens apparatus ofclaim 9, wherein the image light and the other image light mixed by thechromatic separating element are projected to form a color image. 11.The projection lens apparatus of claim 1, wherein the linear reflectingpolarizer is adapted to direct the illumination light substantially in afirst polarization toward the back lens unit and to direct the imagelight substantially in a second polarization.
 12. The projection lensapparatus of claim 1, wherein the imager is adapted to receive theportion of the color separated light substantially in a firstpolarization and to direct the image light substantially in a secondpolarization.
 13. The projection lens apparatus of claim 1, wherein theimage light passes through the linear reflecting polarizer and projectsfrom the front lens to a display screen.
 14. The projection lensapparatus of claim 1, wherein the linear reflecting polarizer comprisesa substantially nonabsorbing polarizer.
 15. The projection lensapparatus of claim 1, wherein the linear reflecting polarizer comprisesmultilayer optical film.
 16. The projection lens apparatus of claim 1,wherein the imager is adapted to direct the image light substantiallyretarded by one-half wave with respect to the portion of the colorseparated light.
 17. The projection lens apparatus of claim 1, furthercomprising an aperture stop adapted to pass the image light and whoseposition is accessible.
 18. The projection lens apparatus of claim 17,wherein the aperture stop is adapted to receive a filter for filteringthe image light.
 19. The projection lens apparatus of claim 1, whereinthe chromatic separating element comprises a Philips prism.
 20. Theprojection lens apparatus of claim 1, wherein the chromatic separatingelement comprises an X cube prism.
 21. The projection lens apparatus ofclaim 1, wherein the field lens comprises a weak, negatively poweredlens.
 22. The projection lens apparatus of claim 1, wherein thechromatic separating element is adapted to recombine the color separatedlight it had previously separated.
 23. The projection lens apparatus ofclaim 1, wherein the chromatic separating element is adapted to separatethe illumination light that is substantially telecentric at the imager.24. The projection lens apparatus of claim 1, wherein the back lens unitand the linearly reflecting polarizer are adapted to direct theillumination light substantially in a first polarization that issubstantially telecentric at the imager.
 25. The projection lensapparatus of claim 1, wherein the front lens unit comprises an asphericsurface adapted to pass the image light.
 26. The projection lensapparatus of claim 1, wherein the front lens unit comprises overallnegative optical power.
 27. The projection lens apparatus of claim 1,wherein the back lens unit comprises overall positive power.
 28. Theprojection lens apparatus of claim 1, wherein the front lens unitcomprises two aspheres adapted to pass the image light.
 29. Theprojection lens apparatus of claim 1, wherein the front lens unitcomprises a compound lens element having a clean-up element adapted toclean-up the image light.
 30. The projection lens apparatus of claim 29,wherein the clean-up element comprises a filter adapted to filter theimage light.
 31. The projection lens apparatus of claim 29, wherein theclean-up element comprises a polarizing filter.
 32. The projection lensapparatus of claim 1, wherein the front lens unit is adapted to pass theimage light substantially in one polarization to a projection screen.33. The projection lens apparatus of claim 1, wherein an aperture stopis disposed between the linear reflecting polarizer and the first lensunit.
 34. The projection lens apparatus of claim 1, wherein the frontlens unit comprises an aspheric surface.
 35. The projection lensapparatus of claim 1, wherein the front lens unit comprises at least oneasphere.
 36. The projection apparatus of claim 1, wherein an entrancepupil of the projection lens is adapted to match an exit pupil of alight source.
 37. The image projection lens system of claim 25, whereinthe projection lens comprises a remote aperture stop.